sched: walt: fix window misalignment when HZ=300

Due to rounding error hrtimer tick interval becomes 3333333 ns when HZ=300.
Consequently the tick time stamp nearest to the WALT's default window size
20ms will be also 19999998 (3333333 * 6).

Change-Id: I08f9bd2dbecccbb683e4490d06d8b0da703d3ab2
Suggested-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>